OF ARTS AND SCIENCES. 65 



often volatilization of molybdic teroxide when a phospho-molybdate is 

 ignited at a temperature sufficient to expel its water. The analyses 

 require great care and no small amount of practice to insure good 

 results. As in the case of the phosj>ho-tungstates, the alkaline bases 

 are best determined by difference. 



Twenty-four Atom Series. — Phospho-molybdic acid. The acid of 

 this series was first obtained by Debray, who prepared it by boiling 

 aramonic phospho-molybdate with nitro-muriatic acid, and allowing 

 the solution to evaporate spontaneously. I find that this is a good 

 method of obtaining the acid, but the following details should be 

 observed. The bright yellow ammonic phospho-molybdate should be 

 first dried, and then heated with a large excess of strong aqua-regia 

 in a casserole over an iron capsule to serve as a radiator. In this 

 manner the decomposition proceeds very regularly and without suc- 

 cussions. When it becomes necessary to add fresh acid, the super- 

 natant liquid should be allowed to settle completely and then be poured 

 off carefully. Fresh acid may then be added, and the process, which 

 is at best a slow one, continued. When the ammonium salt has disap- 

 peared, the liquid is to be evaporated until the excess of nitric and 

 chlorhydric acids has been expelled. On standing, large bright yellow 

 octahedral crystals are obtained from the very concentrated solution. 

 These may be redissolved and recrystallized, but there is always 

 some loss in the process of purification, because solution in water pro- 

 duces more or less decomposition of the acid with formation of a pale 

 greenish white crystalline body. This substance passes very readily 

 through a filter, and the solution of the acid must be allowed to settle 

 completely before the clear supernatant liquid is brought upon the fil- 

 ter. Debray obtained three different hydrates of phospho-molybdic 

 acid, to which he gave, respectively, the formulas 



20 M0O3 . P.O^ . 3 H^O + 21 aq, 



20 M0O3 . F.p, . 3 a,0 -f- 48 aq, 

 and 



20 M0O3 . P.O., . 3 H,0 + 38 aq. 



Unfortunately he has not given either the methods or the com- 

 plete results of his analyses. In the first hydrate he found 13.30^, 

 in the second 23.40^, and in the the third 19.60% of water. 



I obtained the acid only in transparent octahedral crystals which 

 had a bright yellow color. Of these crystals, dried by pressure with 

 woollen paper, 



VOL. XVII. (n. S. IX.) 5 



